There has been hitherto known a Nb capacitor, more particularly, solid electrolytic Nb capacitor in which Nb (niobium) is used for an anode body. An outline of a typical process for preparing the solid electrolytic Nb capacitor will be explained below step by step.
1) Powder-preparing step PA0 2) Compression molding & sintering step PA0 3) Formation step PA0 4) Solid-electrolyte-layer-forming step PA0 4) Steps of forming graphite and Ag layers to mold-application After forming the solid electrolyte layer, graphite and Ag layers are formed on the thus formed solid electrolyte layer in this order. Then, the thus obtained laminate is subjected to mold-application.
Nb powders are admixed with a liquid binder to granulate secondary Nb particles of coagulated Nb powders. These granulated secondary Nb particles are then sieved to eliminate enormously large mass of coagulated secondary Nb particles, thereby, to obtain secondary Nb particles having a uniform particle size.
By using the secondary Nb particles having a uniform particle size which was prepared in the above powder-preparing step, an anode body for a capacitor is made. Explaining it more in detail, lead of Nb wire is embedded into the secondary Nb particles. Then, a compression-molded article (-compact) of the secondary Nb particles is prepared. Thereafter, the prepared compression-molded article is sintered in a vacuum of not more than 10.sup.-5 Torr at high temperatures of 1200 to 1300.degree. C. to prepare an anode body for a capacitor.
On the surface of the anode body for a capacitor prepared in the above compression molding & sintering step, a dielectric of a Nb-oxide layer is formed by an anodic oxidation process. The anodic oxidation process is a process for forming an oxide layer on the surface of an anode body by immersing anode body and its counter electrode in an electrolytic solution and maintaining the anode body at an electrostatic potential higher than that of a counter electrode. Using, for example, an aqueous solution containing 0.6% by volume of phosphoric acid as an electrolytic solution, the anodic oxidation is carried out with keeping its liquid temperature at 60.degree. C.
On the dielectric, i.e., the oxide layer of Nb, a solid electrolyte layer is formed. The solid electrolyte layer is formed by polymerizing a conductive polymer such as polypyrrole or the like.